Field of the Invention
The present invention relates to the production of a corrosion-protection coating on a substrate that has cavities.
Related Art
In the context of intensification of the industrial processes used by the petroleum and chemical industries, the engineering departments concerned seek to improve the efficiency of the units that they design. So as to increase the efficiency, the approach followed during the development of new production units is the replacement, when this is possible, of the exchangers and reactors by devices of structured exchanger or exchanger-reactor type, enabling a great improvement in the yield of the unit. These exchangers or exchanger-reactors make it possible to have: (1) much more effective mass and heat transfers due to their high volume to surface area ratio, (2) a good thermal and structural stability and (3) a finer control of the operating conditions of the process leading to a higher production yield. These devices are usually formed of assembled grooved plates that form millimeter-sized channels.
When this technology is used in hydrocarbon cracking or reforming processes such as for example the steam methane reforming process used for the industrial production of syngas, the walls of the channels of the exchangers or exchanger-reactors must be covered by a protective coating so as to block the catastrophic effects of the high carbon activity that prevails within these processes. These conditions are behind the “metal dusting” phenomena, which are characterized by a disintegration of the alloys into particles of metal and of graphite, thus leading to the degradation of these exchangers or exchanger-reactors if they are not protected.
The protective coatings used may be based on aluminum or on chromium so as to develop protective stable oxide layers of α-Al2O3 or Cr2O3 type. In general, aluminum or chromium is deposited in the gas phase, which operation is also referred to as gas aluminizing or chromizing. The oxide of aluminum or of chromium is then obtained either by a specific oxidation step in a furnace under controlled or uncontrolled partial pressure of oxygen, or by directly using the exchanger or exchanger-reactor in an oxidizing atmosphere.
The vapor deposition techniques use a gaseous precursor of the coating to be produced. This precursor may be produced in direct proximity to the surface to be coated (pack cementation) or be transported via a gas to the surface to be coated (out of pack, CVD using a gas cylinder or mixture, etc.). The main difficulties encountered for pack cementation are linked to the filling of parts that have a complex geometry or very small dimensions (several mm) with the cement powder (precursor mixture of the coating). The main limitations of techniques that use gaseous precursors relate to the rapid depletion of reactive species from the gaseous mixture leading to heterogeneities of chemical composition and/or of thickness of the coating. It is very difficult to obtain a uniform coating over great lengths or on surfaces that are difficult to access.